This invention relates to the use of reinforced thermoplastic materials as a consolidation (pressurizing) medium in the manufacture of composite articles. More particularly, it relates to the molding of high temperature thermoplastic composites in an open mold press.
The manufacture of composite articles usually requires the application of uniform pressure to consolidate the part to produce void-free articles. The pressurizing medium must conform to the finished part surface and evenly distribute the applied pressure. Current techniques involve enclosed molds where one or several sides can move to impart pressure on the molded article. A special case of this is diaphragm forming where the composite article is placed between a deformable diaphragm and a single-sided tool. An entrapped fluid acts upon the diaphragm, applying pressure on the article to both form and consolidate it. When rigid molds are used and the molded material itself is not easily flowable, a compliant surface, such as silicone rubber or soft aluminum, can be provided to aid in redistributing the pressure to make it more uniform.
Molding of advanced thermoplastic composites usually takes place above 370.degree. C. and requires consolidation pressures up to 500 psi or more. Current compliant materials are inadequate in providing the desired effect at these conditions. For example, silicone rubber begins to degrade above 260.degree. C. For open molds, the consolidation pressures can also be high enough to squeeze out the silicone rubber. At the other extreme, pressures required to cause adequate flow in soft aluminum at these temperatures are usually much higher than required for consolidation and pressure is not effectively distributed.
Several factors point to the need for a compliant interlayer to serve as a consolidation medium between the material and the tool.
First, to apply adequate pressure during the consolidation step, the tool surface must closely match the surface of the formed part. Should any gaps exist between the tool and the material, the internal fiber stresses cause the material to puff out to the tool surface, allowing voids to form in the cross section.
Second, even with a perfectly machined tool gap, the material itself can thin during the process creating a gap mismatch. The location of the thinning will be slightly different for each process cycle so the gap cannot simply be predicted and compensated in the machined tool gap.
Third, the material can wrinkle during forming if insufficient tension is applied. Local jamming of the tool gap by the wrinkle can change the tool gap over the entire surface of the part, causing voids elsewhere.